Control unit and control method for reductant supply device

ABSTRACT

There are provided a reductant supply device and a control method for the reductant supply device, which can prevent heat damage of a reductant injection valve, and also prevent crystallization of urea solution due to excessive cooling of the solution reductant. 
     The reductant supply device which is used in an exhaust gas purification device that injects and supplies, as a reductant, a urea solution to an exhaust gas upstream side of a reduction catalyst disposed in an exhaust gas passage of an internal combustion engine, and that reduces and purifies nitrogen oxides contained in exhaust gas using the reduction catalyst, the reductant supply device having a reductant injection valve that is fixed to an exhaust pipe on the exhaust gas upstream side of the reduction catalyst, includes: a cooling water circulation passage that circulates at least part of cooling water of the internal combustion engine to cool the reductant injection valve; flow rate control means for adjusting a flow rate of cooling water flowing through the cooling water circulation passage; temperature detection means for detecting a temperature of the reductant injection valve; and control means for controlling the flow rate control means based on the temperature of the reductant injection valve.

TECHNICAL FIELD

The present invention relates to a reductant supply device and a control method used in an exhaust gas purification device. Particularly, the present invention relates to a reductant supply device that uses a urea solution as a reductant and that cools a reductant injection valve by circulating cooling water for cooling an internal combustion engine, and a control method for the reductant supply device.

BACKGROUND ART

Generally, exhaust gas discharged from an internal combustion engine, such as a diesel engine, contains nitrogen oxides (hereinafter referred to as NO_(x)) that may have an impact on the environment. As an exhaust gas purification device used to purify NO_(x), an exhaust gas purification device (an SCR system) is known which injects and supplies a liquid reductant, such as a urea solution, into exhaust gas on the upstream side of a reduction catalyst disposed in an exhaust gas passage, and which selectively reduces and purifies NO_(x) using the reduction catalyst.

As one form of an exhaust gas purification device structured like that described above, an exhaust gas purification device of a type is known that pressure-feeds a liquid reductant toward a reductant injection valve fixed to an exhaust pipe, and controls opening and closing of the reductant injection valve, thereby injecting and supplying the liquid reductant into the exhaust pipe. In this type of exhaust gas purification device, if a urea solution is used as a liquid reductant and the temperature of the urea solution becomes excessively high, urea is hydrolyzed and crystallized before the urea solution is injected from the reductant injection valve. As a result, there is a possibility that a supply system of the liquid reductant will be partially or completely clogged. In light of this fact, in order to maintain the urea solution at a sufficiently low temperature, a method for reducing NO_(x) discharge has been proposed in which a heat exchange fluid, such as an engine coolant, is caused to pass through a line or an injector when heat is exchanged, while maintaining sufficient supply speed of the urea solution and a heat exchange fluid channel (see Patent Document 1).

Further, an electromagnetically controlled valve that is used as the above-described reductant injection valve is directly attached to an exhaust pipe, regardless of the fact that a control part and a plastic part of the valve are relatively weak against heat. As a result, the electromagnetically controlled valve is likely to suffer from heat damage caused by heat being transferred from the exhaust pipe.

To address this, although a hydrocarbon (HC) fuel is used as a reductant, an exhaust gas purification device for a diesel engine has been proposed that is capable of improving durability of an injector for adding a reductant to a NO_(x) catalyst. More specifically, an exhaust gas purification device for a diesel engine is disclosed that includes a NO_(x) catalyst arranged in an exhaust gas passage of the engine, and an injector for adding a NO_(x) reductant that is provided in the exhaust gas passage on the upstream side of the NO_(x) catalyst. The exhaust gas purification device includes: a cooling water passage provided in the injector; a circulation passage that connects the cooling water passage and an engine cooling water passage; and circulation means for circulating cooling water between the cooling water passage and an engine cooling water passage via the circulation passage (see Patent Document 2).

Patent Document 1: Published Japanese Translation of PCT Application No. JP-T-2001-518830 (Claim 11, page 8, lines 7 to 10)

Patent Document 2: Japanese Patent Application Publication No. JP-A-9-96212 (full text, all drawings)

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

When the reductant injection valve is cooled by circulating engine cooling water, the reductant injection valve is cooled effectively because a relatively large amount of the engine cooling water is circulated. However, in some cases, the reductant injection valve is cooled more than necessary due to a large circulation amount of the cooling water. As a result, not only crystallization of urea that is caused because the temperature of the urea solution becomes high and the urea solution is decomposed in a reductant supply device as indicated in Patent Document 1, but also crystallization of urea due to natural evaporation of a solvent may occur in the vicinity of a nozzle hole of the reductant injection valve.

More specifically, because the engine cooling water is maintained at approximately 70 to 80° C., for example, if the circulation amount of the cooling water that is circulated in the reductant injection valve becomes large, the temperature of the reductant injection valve may be cooled to approximately 80 to 100° C., for example. On the other hand, a urea aqueous solution that flows into the reductant supply valve is fed to the nozzle hole while being maintained at a temperature less than 100° C. even if the urea aqueous solution is affected by the heat of the reductant injection valve. Accordingly, the temperature of the urea aqueous solution injected from the reductant injection valve becomes lower than the boiling point in the exhaust gas passage whose pressure is relatively close to the atmospheric pressure. Therefore, the urea aqueous solution does not rapidly evaporate, and is likely to adhere to the vicinity of the nozzle hole. The urea aqueous solution that is maintained at a temperature lower than 100° C. is likely to cause precipitation due to natural evaporation of water acting as a solvent, in an environment under a pressure relatively close to atmospheric pressure. In addition, an exhaust gas flow occurs in the vicinity of the nozzle hole of the reductant invention valve. As a result, natural evaporation of the water contained in the urea aqueous solution is more likely to occur, and precipitation of urea is facilitated. If crystallization of urea occurs in the vicinity of the nozzle hole of the reductant injection valve in this manner, the atomization of the urea aqueous solution is adversely affected. At the same time, clogging of the nozzle hole may occur.

To address this, the inventors of the present invention have made strenuous efforts, and have found that the above-described problems can be solved by providing means for controlling the circulation amount of cooling water, in a reductant supply device having a structure in which engine cooling water is circulated to cool a urea solution. Thus, the present invention has been achieved. More specifically, it is an object of the present invention to provide a reductant supply device which can prevent heat damage of a reductant injection valve by adjusting the circulation amount of cooling water in accordance with the temperature of the reductant injection valve, and which can also prevent crystallization of a urea solution due to excessive cooling of a liquid reductant, and a control method for the reductant supply device.

Means for Solving the Problems

In order to solve the problems described above, according to the present invention, there is provided a reductant supply device which is used in an exhaust gas purification device that injects and supplies, as a reductant, a urea solution to an exhaust gas upstream side of a reduction catalyst disposed in an exhaust gas passage of an internal combustion engine, and that reduces and purifies nitrogen oxides contained in exhaust gas using the reduction catalyst, the reductant supply device having a reductant injection valve that is fixed to an exhaust pipe on the exhaust gas upstream side of the reduction catalyst. The reductant supply device is characterized by including: a cooling water circulation passage that circulates at least part of cooling water of the internal combustion engine to cool the reductant injection valve; flow rate control means for adjusting a flow rate of cooling water flowing through the cooling water circulation passage; temperature detection means for detecting a temperature of the reductant injection valve; and control means for controlling the flow rate control means based on the temperature of the reductant injection valve.

Further, with the structure of the reductant supply device of the present invention, it is desirable that the control means controls the flow rate control means such that the temperature of the reductant injection valve is maintained at a temperature lower than a heat resistance temperature of the reductant injection valve.

Further, with the structure of the reductant supply device of the present invention, it is desirable that the control means controls the flow rate control means such that the temperature of the reductant injection valve is maintained at a temperature equal to or higher than a boiling point of the urea solution.

Further, with the structure of the reductant supply device of the present invention, it is desirable that the temperature detection means calculates a temperature at a tip of the reductant injection valve based on at least one of a temperature of the exhaust gas, a flow rate of the exhaust gas, a temperature of the liquid reductant, a temperature of the cooling water, an outside air temperature, and an injection supply amount from the reductant injection valve.

Further, with the structure of the reductant supply device of the present invention, if the cooling water circulation passage, the flow rate control means and the control means are respectively referred to as a first cooling water circulation passage, first flow rate control means and first control means, it is desirable that the reductant supply device further includes: a second cooling water circulation passage that circulates at least part of the cooling water of the internal combustion engine in order to adjust a temperature of the urea solution in a storage tank that stores the urea solution; second flow rate control means for adjusting a flow rate of the cooling water that flows through the second cooling water circulation passage; and second control means for controlling the second flow rate control means based on the temperature of the urea solution in the storage tank.

Furthermore, according to another aspect of the present invention, there is provided a control method for a reductant supply device which is used in an exhaust gas purification device that injects and supplies a liquid reductant to an exhaust gas upstream side of a reduction catalyst disposed in an exhaust gas passage of an internal combustion engine, and that reduces and purifies nitrogen oxides contained in exhaust gas using the reduction catalyst, the reductant supply device having a reductant injection valve that is fixed to an exhaust pipe on the exhaust gas upstream side of the reduction catalyst. The control method for the reductant supply device is characterized in that: the reductant injection valve is cooled by circulating at least part of cooling water of the internal combustion engine; a temperature of the reductant injection valve is detected; and a flow rate of the cooling water is controlled such that a temperature of the reductant injection valve is maintained in a predetermined range.

ADVANTAGE OF THE INVENTION

According to the reductant supply device of the present invention, the reductant injection valve is cooled effectively by using the cooling water of the internal combustion engine. Further, while the temperature at the tip of the reductant injection valve is detected, the flow rate of the cooling water is adjusted in accordance with the detected temperature. As a result, it is possible to prevent excessive cooling of the reductant injection valve. Accordingly, heat damage of the reductant injection valve is prevented, and crystallization of urea in the vicinity of the reductant injection valve is prevented. Thus, stable atomization of the reductant is realized.

Further, in the reductant supply device of the present invention, the control means performs control such that the temperature of the reductant injection valve is maintained at a temperature lower than the heat resistance temperature. Thus, heat damage of the reductant injection valve is reliably prevented.

Further, in the reductant supply device of the present invention, the control means performs control such that the temperature of the reductant injection valve is maintained at a temperature equal to or higher than a boiling point of the urea solution. Thus, the urea solution that is exposed in the exhaust pipe is also rapidly vaporized due to exhaust heat, and crystallization of urea in the vicinity of the nozzle hole is reliably inhibited.

Further, in the reductant supply device of the present invention, the temperature detection means is means for calculating the temperature at the tip of the reductant injection valve. Therefore, it is possible to estimate the temperature at the tip of the reductant injection valve by using a known device structure, without using an additional temperature sensor.

Further, in the reductant supply device of the present invention, the cooling water of the internal combustion engine is used to adjust the temperature of the urea solution in the storage tank. Therefore, the urea solution having a temperature equal to or lower than the boiling point is inhibited from being injected without adjustment from the reductant injection valve. As a result, crystallization of urea in the vicinity of the nozzle hole is inhibited more reliably.

Moreover, according to the control method for the reductant supply device of the present invention, when the reductant injection valve is cooled by circulating the cooling water of the internal combustion engine, the circulation amount of the cooling water is adjusted based on the temperature of the reductant injection valve. Thus, heat damage of the reductant injection valve is prevented, and crystallization of urea due to excessive cooling of the reductant injection valve is prevented. Accordingly, stable atomization of the reductant is realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of the structure of an exhaust gas purification device.

FIG. 2 is a block diagram showing an example of the structure of a control unit (DCU) of a reductant supply device provided in the exhaust gas purification device.

FIG. 3 is a flowchart showing an example of temperature control of a reductant injection valve that uses cooling water of an internal combustion engine.

FIG. 4 is a flowchart showing an example of temperature control of a urea aqueous solution that uses the cooling water of the internal combustion engine.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment relating to a reductant supply device and a control method for the reductant supply device of the present invention will be described concretely with reference to the appended drawings. However, the embodiment is just one form of the present invention and in no way limits the present invention, and any modification can be made within the scope of the present invention.

Note that, in the respective drawings, structural members that are the same are denoted with the same reference numerals, and explanation thereof is omitted as appropriate.

1. Exhaust Gas Purification Device

First, an example of the structure of an exhaust gas purification device in which a reductant supply device of the present embodiment is provided will be described with reference to FIG. 1.

An exhaust gas purification device 10 shown in FIG. 1 injects and supplies a urea aqueous solution serving as a liquid reductant to the upstream side of a reduction catalyst 13 disposed in an exhaust gas passage. The exhaust gas purification device 10 selectively reduces and purifies NO_(x) contained in exhaust gas using the reduction catalyst 13. The exhaust gas purification device 10 includes, as main elements, the reduction catalyst 13 and a reductant supply device 20. The reduction catalyst 13 is arranged in the middle of an exhaust pipe 11 that is connected to an internal combustion engine 5, and selectively reduces NO_(x) contained in exhaust gas. The reductant supply device 20 includes a reductant injection valve 31 that injects and supplies the urea aqueous solution into the exhaust pipe 11, on the upstream side of the reduction catalyst 13.

2. Reductant Supply Device

The reductant supply device 20 provided in the exhaust gas purification device 10 of the present embodiment includes: the reductant injection valve 31 that is fixed to the exhaust pipe 11 on the upstream side of the reduction catalyst 13; a storage tank 50 that stores the urea aqueous solution; a pump module 40 having a pump 41 that pressure-feeds the urea aqueous solution from the storage tank 50 to the reductant injection valve 31; and a control unit (hereinafter referred to as a “DCU: dosing control unit”) 60 that performs control of the reductant injection valve 31 and the pump 41 in order to control an injection amount of the reductant that is injected and supplied into the exhaust pipe 11. Further, the pump module 40 and the reductant injection valve 31 are connected by a first supply passage 58. The storage tank 50 and the pump module 40 are connected by a second supply passage 57. Furthermore, the pump module 40 and the storage tank 50 are connected by a circulation passage 59.

For example, an on-off valve whose on-off positioning is controlled by duty control is used as the reductant injection valve 31. The urea aqueous solution that is pressure-fed from the pump module 40 to the reductant injection valve 31 is maintained at a predetermined pressure. When the reductant injection valve 31 is opened by a control signal transmitted from the DCU 60, the urea aqueous solution is injected into the exhaust gas passage.

Further, a cooling water passage 37 is provided in the reductant injection valve 31, and the cooling water of the internal combustion engine 5 is used to cool the reductant injection valve 31. In the example of the reductant supply device 20 of the present embodiment, a first cooling water circulation passage 85 that includes the cooling water passage 37 of the reductant injection valve 31 is provided. The cooling water of the internal combustion engine 5 is circulated by a cooling water circulation pump 73 through a cooling water passage 75 of the internal combustion engine 5, and diverges from the cooling water passage 75 and also flows into the first cooling water circulation passage 85. The cooling water that has flowed into the first cooling water circulation passage 85 returns again to the cooling water passage 75 of the internal combustion engine 5, by way of the cooling water passage 37 provided in the reductant injection valve 31. Thus, the reductant injection valve 31 is cooled.

A first cooling water flow rate control valve 81 for adjusting the flow rate of the cooling water flowing through the first cooling water circulation passage 85 is provided in the first cooling water circulation passage 85 on the upstream side of the reductant injection valve 31. For example, an on-off valve of an electromagnetically controlled type, an electromagnetic proportional flow rate control valve or the like is used as the first cooling water flow rate control valve 81, and is controlled by the DCU 60 which will be described later. Normally, the first cooling water flow rate control valve 81 is opened, and the reductant injection valve 31 is cooled by the cooling water. On the other hand, when there is a possibility that the reductant injection valve 31 will be cooled excessively, the first cooling water flow rate control valve 81 is closed to block the circulation of the cooling water, or the opening degree of the first cooling water flow rate control valve 81 is reduced to decrease the flow rate of the cooling water. The reductant injection valve 31 is thereby adjusted so as not to be cooled excessively.

Further, in the first supply passage 58 connected to the reductant injection valve 31, an inlet portion of the reductant injection valve 31 is provided with a temperature sensor 33 for detecting the temperature of the urea aqueous solution that flows into the reductant injection valve 31. Further, in the first cooling water circulation passage 85, an inlet portion on the upstream side of the reductant injection valve 31 is provided with a temperature sensor 35 for detecting the temperature of the cooling water that flows into the reductant injection valve 31. Temperature information detected by the sensors 33 and 35 is transmitted to the DCU 60.

Moreover, in the reductant supply device 20 of the present embodiment, a second cooling water circulation passage 87 is provided such that it further diverges, on the upstream side of the first cooling water flow rate control valve 81, from the first cooling water circulation passage 85 that diverges from the cooling water passage 75 of the internal combustion engine 5. The second cooling water circulation passage 87 is arranged such that it passes through the storage tank 50, and joins again with the first cooling water circulation passage 85. A second cooling water flow rate control valve 83 for adjusting the flow rate of the cooling water flowing through the second cooling water circulation passage 87 is provided in the second cooling water circulation passage 87 on the upstream side of the storage tank 50.

The cooling water of the internal combustion engine 5 that circulates through the second cooling water circulation passage 87 is used as heating means for heating the urea aqueous solution in the storage tank 50. The cooling water of the internal combustion engine 5 is maintained at a temperature of approximately 70 to 80° C., for example. Therefore, when the temperature of the urea aqueous solution in the storage tank 50 decreases, the second cooling water flow rate control valve 83 is opened to circulate the cooling water in the storage tank 50. Thus, control is performed such that the temperature of the urea aqueous solution does not excessively decrease and the urea aqueous solution does not freeze.

In the same manner as in the first cooling water flow rate control valve 81, an on-off valve of an electromagnetically controlled type, an electromagnetic proportional flow rate control valve or the like is used as the second cooling water flow rate control valve 83, and is controlled by the DCU 60. More specifically, a temperature sensor 51 for detecting the temperature of the urea aqueous solution in the tank is provided in the storage tank 50 that stores the urea aqueous solution. The value detected by the temperature sensor 51 is output to the DCU 60 as a signal, and the second cooling water flow rate control valve 83 is controlled based on the temperature information.

Further, the pump module 40 is provided with the pump 41. The pump 41 pumps the urea aqueous solution in the storage tank 50 via the second supply passage 57, and pressure-feeds the urea aqueous solution to the reductant injection valve 31 via the first supply passage 58. The pump 41 is, for example, an electric gear pump, and can be designed to be duty-controlled by a signal transmitted from the DCU 60. Further, a pressure sensor 43 is provided in the first supply passage 58, and the value detected by the pressure sensor 43 is output to the DCU 60 as a signal. The drive duty of the pump 41 is controlled such that the pressure value in the first supply passage 58 is maintained at a predetermined value. More specifically, in a state where the pressure in the first supply passage 58 becomes lower than the predetermined value, the drive duty of the pump 41 is controlled to be increased. Conversely, in a state where the pressure in the first supply passage 58 becomes higher than the predetermined value, the drive duty of the pump 41 is controlled to be reduced.

Note that, the term “drive duty of the pump” means the ratio of a pump drive time taken in one cycle, in a pulse width modulation (PWM) control.

In addition, a main filter 47 is provided in the first supply passage 58, and foreign matter contained in the urea aqueous solution that is pressure-fed to the reductant injection valve 31 is caught. Further, the circulation passage 59 is provided such that it diverges from the first supply passage 58 between the pump 41 and the main filter 47, and the circulation passage 59 is connected to the storage tank 50. An orifice 45 is provided in the middle of the circulation passage 59, and a pressure control valve 49 is provided closer to the storage tank 50 than the orifice 45. With the provision of the circulation passage 59 structured as described above, when the pressure value in the first supply passage 58 exceeds the predetermined value in a state where the urea aqueous solution is pressure-fed by the pump 41 that is feedback controlled based on the detection value of the pressure sensor 43, the pressure control valve 49 is opened and a part of the urea aqueous solution flows back into the storage tank 50. For example, a known check valve or the like can be used as the pressure control valve 49.

Further, a reverting valve 71 is provided in the pump module 40. When the reductant supply device 20 does not perform injection control of the urea aqueous solution, the urea aqueous solution in a reductant supply passage, which includes the pump module 40, the reductant injection valve 31, the first supply passage 58 and the second supply passage 57, is collected into the storage tank 50. Therefore, when the internal combustion engine 5 is stopped and the control of the reductant supply device 20 is not performed in a cold condition, i.e., under a temperature condition in which the urea aqueous solution is likely to freeze, freezing of the urea aqueous solution in the reductant supply passage is prevented. When the operation of the internal combustion engine is restarted after that, it is ensured that an injection failure due to clogging does not occur.

The reverting valve 71 is, for example, a switching valve that functions to switch a flow channel of the urea aqueous solution, from a forward direction, i.e., a direction from the storage tank 50 toward the pump module 40, to an opposite direction, i.e., a direction away from the pump module 40 toward the storage tank 50. When an ignition switch of the internal combustion engine is turned off, if the flow channel is switched to the opposite direction, the urea aqueous solution is collected in the storage tank 50.

Moreover, heaters 92 to 97 are provided in respective sections of the reductant supply passage in the reductant supply device 20. The heaters 92 to 97 are provided in order to prevent a case where, if the urea aqueous solution is present in the reductant supply passage in a cold condition, the urea aqueous solution freezes and clogs the reductant supply passage partially or completely, thus inhibiting the reductant injection control from being performed accurately by the reductant injection valve 31. Further, energization of the heaters 92 to 97 is controlled by the DCU 60. When it is determined that the reductant supply passage is under a temperature condition in which freezing of the urea aqueous solution may occur, based on, for example, the temperature of the urea aqueous solution, the outside air temperature or the like, voltage is supplied from a battery and heating is performed.

The heaters 92 to 97 are not particularly limited, and heating wires or the like can be used.

3. Control Unit (DCU) for the Reductant Supply Device

FIG. 2 shows the structure of the DCU 60 for controlling the reductant supply device of the present embodiment. The DCU 60 is principally structured by a microcomputer having a known structure. FIG. 2 shows an example of the structure that is represented by functional blocks that correspond to portions relating to operation control of the first cooling water flow rate control valve 81 and the second cooling water flow rate control valve 83 shown in FIG. 1.

The DCU 60 of the present embodiment includes, as main elements, an injection valve temperature detection portion (denoted by “Injector-Temp Detection”) that detects the temperature of the reductant injection valve, a first control portion (denoted by “Injector-Cooling Control”) that controls the first cooling water flow rate control valve, and a second control portion (denoted by “Tank-Heating Control”) that controls the second cooling water flow rate control valve. These portions are specifically realized by the microcomputer (not shown in the figures) executing a program.

Among these portions, the injection valve temperature detection portion detects the temperature of the reductant injection valve, and transmits temperature information to the first control portion. A method for detecting the temperature of the reductant injection valve is not particularly limited. The temperature may be directly detected by providing a temperature sensor on the reductant injection valve, or may be estimated by calculation.

The injection valve temperature detection portion in the DCU 60 of the present embodiment estimates a temperature Ti at the tip of the reductant injection valve, based on respective pieces of information, namely, an exhaust gas temperature Tg and an exhaust gas flow rate Ve that are estimated from an operation state of the internal combustion engine, a temperature Tu1 of the urea aqueous solution in the storage tank, a temperature Tu2 of the urea aqueous solution that flows into the reductant injection valve, a temperature Tr of the cooling water that flows into the reductant injection valve, an outside air temperature To, an injection command value Qu transmitted from the DCU 60 to the reductant injection valve, and a vehicle speed S.

The temperature Ti at the tip of the reductant injection valve is estimated by the injection valve temperature detection portion. This is because the flow rate of the cooling water can be controlled based on a temperature in the vicinity of a nozzle hole where crystallization of urea is likely to occur due to evaporation of water contained in the urea aqueous solution. Moreover, the reductant injection valve is heated by exhaust heat that is transferred via the exhaust pipe. Therefore, if the temperature at the tip of the reductant injection valve, at which the temperature becomes highest, is maintained at a temperature equal to or lower than a heat resistance temperature, it is possible to maintain the entire reductant injection valve at a temperature equal to or lower than the heat resistance temperature.

Note that, when the temperature Ti at the tip of the reductant injection valve is estimated, all the information described above need not necessarily be referred to, and the estimation may be performed based on only part of the information.

The first control portion, which performs control of the first cooling water flow rate control valve, outputs a control signal for controlling the opening and closing of the first cooling water flow rate control valve, based on the temperature Ti at the tip of the reductant injection valve that is transmitted from the injection valve temperature detection portion.

For example, when the internal combustion engine is normally operated, in order to maintain the temperature Ti at the tip of the reductant injection valve at a temperature lower than the heat resistance temperature, the first cooling water flow rate control valve is opened to circulate a relatively large amount of cooling water through the first cooling water circulation passage 85. The temperature Ti at the tip of the reductant injection valve that is necessary to maintain a plastic portion and an electromagnetically controlled portion of the reductant injection valve at a temperature equal to or lower than the heat resistance temperature varies depending on the type of the reductant injection valve. However, the cooling water is circulated such that the temperature Ti at the tip is maintained at a temperature equal to or higher than 140 and lower than 160° C., for example.

On the other hand, if the reductant injection valve is excessively cooled by the cooling water, crystallization of urea may occur in the vicinity of the nozzle hole. Therefore, the cooling water is blocked by closing the first cooling water flow rate control valve, or the opening degree is decreased to reduce the flow rate of the cooling water. For example, in order to facilitate rapid evaporation and vaporization of the urea aqueous solution that is injected through the nozzle hole, the flow rate of the cooling water is adjusted such that the temperature Ti at the tip of the reductant injection valve is maintained at a temperature equal to or higher than the boiling point of the urea aqueous solution. Although the boiling point of the urea aqueous solution varies depending on concentration, the flow rate of the cooling water is adjusted such that the temperature Ti at the tip is maintained at a temperature equal to or higher than 100 and lower than 110° C., for example.

More specifically, based on information about the temperature Ti at the tip of the reductant injection valve that is transmitted from the injection valve temperature detection portion, the first control portion in the DCU 60 of the present embodiment performs feedback control of the opening and closing of the first cooling water flow rate control valve so that the temperature Ti at the tip of the reductant injection valve becomes equal to or higher than the boiling point of the urea aqueous solution in the exhaust gas passage and lower than the heat resistance temperature of the reductant injection valve. As a result, heat damage of the reductant injection valve is inhibited and durability is thereby improved. At the same time, crystallization of urea in the vicinity of the nozzle hole is inhibited, which allows stable atomization of the urea aqueous solution.

Note that, in the example of the present embodiment, control is performed such that the temperature Ti at the tip of the reductant injection valve is equal to or higher than the boiling point of the urea aqueous solution. However, as long as the urea aqueous solution is rapidly vaporized after injection by the influence of exhaust heat, the temperature Ti at the tip may be lower than the boiling point of the urea aqueous solution.

Further, the second control portion outputs a control signal for controlling the opening and closing of the second cooling water flow rate control valve, based on the temperature of the urea aqueous solution that is detected by the temperature sensor provided in the storage tank.

For example, when the temperature of the urea aqueous solution in the storage tank becomes lower than the freezing point, the urea aqueous solution freezes and cannot be supplied to the reductant injection valve. In addition, even when the temperature of the urea aqueous solution exceeds the freezing point, if it is, for example, 80° C. or higher, the urea aqueous solution may deteriorate.

Given this, when the temperature of the urea aqueous solution in the storage tank is lower than 60° C., the second control portion in the DCU 60 of the present embodiment opens the second cooling water flow rate control valve. Thus, the urea aqueous solution in the storage tank is heated, and freezing of the urea aqueous solution is prevented. On the other hand, when the temperature of the urea aqueous solution in the storage tank is equal to or higher than 60° C., the second cooling water flow rate control valve is closed. Thus, heating by the cooling water of the internal combustion engine is stopped, and deterioration of the urea aqueous solution in the storage tank is prevented.

4. Reductant Injection Control

Next, reductant injection control that is performed by the reductant supply device 20 provided in the exhaust gas purification device 10 shown in FIG. 1 will be described.

When the internal combustion engine is operated, the liquid reductant in the storage tank 50 is pumped by the pump 41, and is pressure-fed to the reductant injection valve 31. At this time, a detection value detected by the pressure sensor 43, which is provided in the first supply passage 58 on the downstream side of the pump 41, is fed back and controlled to indicate a predetermined pressure value. For example, when the detection value is less than the predetermined value, the output of the pump 41 is increased. On the other hand, when the pressure value exceeds the predetermined value, the output of the pump 41 is reduced. At the same time, the fluid reductant is caused to flow back to the storage tank 50 via the pressure control valve 49, and the pressure is reduced. Thus, the pressure of the reductant that is pressure-fed to the reductant injection valve 31 side is maintained at an approximately fixed value.

Further, the reductant that is pressure-fed from the pump module 40 to the reductant injection valve 31 is maintained at an approximately fixed pressure value, and is injected into the exhaust gas passage when the reductant injection valve 31 is opened. The DCU 60 determines an injection supply amount of the reductant to be injected, based on information about the operation state of the internal combustion engine, the temperature of the reduction catalyst 13, and the amount of NO_(x) that has passed through the reduction catalyst 13 without being reduced, the amount of NO_(x) being measured on the downstream side of the reduction catalyst 13. The DCU 60 generates a control signal in accordance with the determined injection supply amount, and outputs it to the reductant injection valve 31. The reductant injection valve 31 is duty controlled by the control signal, and an appropriate amount of reductant is injected and supplied into the exhaust gas passage. The reductant that has been injected into the exhaust gas passage flows into the reduction catalyst 13, and is used in reductive reaction of NO_(x) contained in exhaust gas.

5. Cooling Water Circulation Control

Next, an example of the routine of cooling water circulation control performed by the control unit (DCU) 60 for the reductant supply device of the present embodiment shown in FIG. 2 will be described, with reference to the control flow shown in FIG. 3 and FIG. 4.

First, as shown in FIG. 3, at step S1 after the start, the temperature of the reductant injection valve is detected. As described earlier, in the DCU 60 of the present embodiment, the temperature Ti at the tip of the reductant injection valve is calculated based on the respective pieces of information, namely, the exhaust gas temperature Tg, the exhaust gas flow rate Ve, the temperature Tu1 of the urea aqueous solution in the storage tank, the temperature Tu2 of the urea aqueous solution that flows into the reductant injection valve, the temperature Tr of the cooling water that flows into the reductant injection valve, the outside air temperature To, the injection command value Qu transmitted from the DCU 60 to the reductant injection valve, and the vehicle speed S.

Next, at step S2, it is determined whether or not the detected temperature Ti at the tip of the reductant injection valve is lower than a lower limit value Ti1. In the present embodiment, the lower limit value Ti1 is set to the boiling point of the urea aqueous solution. When the temperature Ti at the tip of the reductant injection valve is lower than the lower limit value Ti1, the process proceeds to step S3, and the first cooling water flow rate control valve is fully closed or its opening degree is reduced. After that, the process returns to the start. As a result, the flow rate of the cooling water that flows through the cooling water passage provided in the reductant injection valve is reduced, and the temperature Ti at the tip of the reductant injection valve is raised by the influence of exhaust heat transferred through the exhaust pipe.

On the other hand, at step 2, when the temperature Ti at the tip of the reductant injection valve is equal to or higher than the lower limit value Ti1, the process proceeds to step S4, and it is determined whether or not the temperature Ti at the tip of the reductant injection valve is equal to or higher than an upper limit value Ti2. In the present embodiment, the upper limit value Ti2 is set to the heat resistance temperature of the reductant injection valve. When the temperature Ti at the tip of the reductant injection valve is lower than the upper limit value Ti2, the process returns to the start without performing any other processing. On the other hand, when the temperature Ti at the tip of the reductant injection valve is equal to or higher than the upper limit value Ti2, the process proceeds to step S5, and the first cooling water flow rate control valve is fully opened or its opening degree is increased. After that, the process returns to the start. As a result, the flow rate of the cooling water that flows through the cooling water passage provided in the reductant injection valve is increased, and the reductant injection valve is cooled and the temperature Ti at the tip thereof decreases.

In the control method for the reductant supply device of the present embodiment, along with the above-described temperature control of the reductant injection valve that is performed by controlling the opening and closing of the first cooling water flow rate control valve, temperature control of the urea aqueous solution in the storage tank is performed by controlling the opening and closing of the second cooling water flow rate control valve.

FIG. 4 shows the flow of the temperature control of the urea aqueous solution in the storage tank. First, at step S11, the temperature Tu1 of the urea aqueous solution in the storage tank is detected. In the case of the reductant supply device of the present embodiment, temperature information that is detected by the temperature sensor provided in the storage tank is read.

Next, at step S12, it is determined whether or not the detected temperature Tu1 of the urea aqueous solution is equal to or lower than a reference value Tu0. The reference value Tu0 at this time is set to 60° C., for example, using as an indication the temperature at which the urea aqueous solution in the storage tank is maintained without deterioration.

When the temperature Tu1 of the urea aqueous solution is equal to or lower than the reference value Tu0, the process proceeds to step S13, and the second cooling water flow rate control valve is fully opened or its opening degree is increased. After that, the process returns to the start. As a result, the urea aqueous solution in the storage tank is heated by the cooling water of the internal combustion engine that is maintained at 70 to 80° C.

On the other hand, when the detected temperature Tu1 of the urea aqueous solution exceeds the reference value Tu0, the temperature of the urea aqueous solution in the storage tank has been raised excessively and the urea aqueous solution may deteriorate. Therefore, the process proceeds to step S14, and the second cooling water flow rate control valve is fully closed or its opening degree is reduced. After that, the process returns to the start.

In this manner, because the temperature of the urea aqueous solution in the storage tank is controlled to be maintained, for example, within a range of 60 to 80° C., freezing and deterioration of the urea aqueous solution are prevented. At the same time, because the urea aqueous solution is rapidly vaporized when it is injected from the reductant injection valve, the urea aqueous solution is easily dispersed uniformly in exhaust gas.

Note that, in the example of the opening and closing control of the second cooling water flow rate control valve according to the present embodiment, it is only determined whether or not the temperature Tu1 of the urea aqueous solution exceeds the reference value Tu0, and then the flow rate control is performed. However, the second reference value Tu2 that is different from the reference value Tu0 may be further set, and the temperature range of the urea aqueous solution in the storage tank may be more finely set. Then, cooling control of the urea aqueous solution in the storage tank may be performed using the cooling water of the internal combustion engine. 

1-6. (canceled)
 7. A reductant supply device which is used in an exhaust gas purification device that injects and supplies, as a reductant, a urea solution to an exhaust gas upstream side of a reduction catalyst disposed in an exhaust gas passage of an internal combustion engine, and that reduces and purifies nitrogen oxides contained in exhaust gas using the reduction catalyst, the reductant supply device having a reductant injection valve that is fixed to an exhaust pipe on the exhaust gas upstream side of the reduction catalyst, the reductant supply device comprising: a cooling water circulation passage that circulates at least part of cooling water of the internal combustion engine to cool the reductant injection valve; flow rate control means for adjusting a flow rate of cooling water flowing through the cooling water circulation passage; temperature detection means for detecting a temperature of the reductant injection valve; and control means for controlling the flow rate control means based on the temperature of the reductant injection valve.
 8. The reductant supply device according to claim 7, wherein the control means controls the flow rate control means such that the temperature of the reductant injection valve is maintained at a temperature lower than a heat resistance temperature of the reductant injection valve.
 9. The reductant supply device according to claim 7, wherein the control means controls the flow rate control means such that the temperature of the reductant injection valve is maintained at a temperature equal to or higher than a boiling point of the urea solution.
 10. The reductant supply device according to claim 8, wherein the control means controls the flow rate control means such that the temperature of the reductant injection valve is maintained at a temperature equal to or higher than a boiling point of the urea solution.
 11. The reductant supply device according to claim 7, wherein the temperature detection means calculates a temperature at a tip of the reductant injection valve based on at least one of a temperature of the exhaust gas, a flow rate of the exhaust gas, a temperature of the liquid reductant, a temperature of the cooling water, an outside air temperature, and an injection supply amount from the reductant injection valve.
 12. The reductant supply device according to claim 8, wherein the temperature detection means calculates a temperature at a tip of the reductant injection valve based on at least one of a temperature of the exhaust gas, a flow rate of the exhaust gas, a temperature of the liquid reductant, a temperature of the cooling water, an outside air temperature, and an injection supply amount from the reductant injection valve.
 13. The reductant supply device according to claim 9, wherein the temperature detection means calculates a temperature at a tip of the reductant injection valve based on at least one of a temperature of the exhaust gas, a flow rate of the exhaust gas, a temperature of the liquid reductant, a temperature of the cooling water, an outside air temperature, and an injection supply amount from the reductant injection valve.
 14. The reductant supply device according to claim 10, wherein the temperature detection means calculates a temperature at a tip of the reductant injection valve based on at least one of a temperature of the exhaust gas, a flow rate of the exhaust gas, a temperature of the liquid reductant, a temperature of the cooling water, an outside air temperature, and an injection supply amount from the reductant injection valve.
 15. The reductant supply device according to claim 7, wherein if the cooling water circulation passage, the flow rate control means and the control means are respectively referred to as a first cooling water circulation passage, first flow rate control means and first control means, the reductant supply device further comprising: a second cooling water circulation passage that circulates at least part of the cooling water of the internal combustion engine in order to adjust a temperature of the urea solution in a storage tank that stores the urea solution; second flow rate control means for adjusting a flow rate of the cooling water that flows through the second cooling water circulation passage; and second control means for controlling the second flow rate control means based on the temperature of the urea solution in the storage tank.
 16. The reductant supply device according to claim 8, wherein if the cooling water circulation passage, the flow rate control means and the control means are respectively referred to as a first cooling water circulation passage, first flow rate control means and first control means, the reductant supply device further comprising: a second cooling water circulation passage that circulates at least part of the cooling water of the internal combustion engine in order to adjust a temperature of the urea solution in a storage tank that stores the urea solution; second flow rate control means for adjusting a flow rate of the cooling water that flows through the second cooling water circulation passage; and second control means for controlling the second flow rate control means based on the temperature of the urea solution in the storage tank.
 17. The reductant supply device according to claim 9, wherein if the cooling water circulation passage, the flow rate control means and the control means are respectively referred to as a first cooling water circulation passage, first flow rate control means and first control means, the reductant supply device further comprising: a second cooling water circulation passage that circulates at least part of the cooling water of the internal combustion engine in order to adjust a temperature of the urea solution in a storage tank that stores the urea solution; second flow rate control means for adjusting a flow rate of the cooling water that flows through the second cooling water circulation passage; and second control means for controlling the second flow rate control means based on the temperature of the urea solution in the storage tank.
 18. The reductant supply device according to claim 10, wherein if the cooling water circulation passage, the flow rate control means and the control means are respectively referred to as a first cooling water circulation passage, first flow rate control means and first control means, the reductant supply device further comprising: a second cooling water circulation passage that circulates at least part of the cooling water of the internal combustion engine in order to adjust a temperature of the urea solution in a storage tank that stores the urea solution; second flow rate control means for adjusting a flow rate of the cooling water that flows through the second cooling water circulation passage; and second control means for controlling the second flow rate control means based on the temperature of the urea solution in the storage tank.
 19. The reductant supply device according to claim 11, wherein if the cooling water circulation passage, the flow rate control means and the control means are respectively referred to as a first cooling water circulation passage, first flow rate control means and first control means, the reductant supply device further comprising: a second cooling water circulation passage that circulates at least part of the cooling water of the internal combustion engine in order to adjust a temperature of the urea solution in a storage tank that stores the urea solution; second flow rate control means for adjusting a flow rate of the cooling water that flows through the second cooling water circulation passage; and second control means for controlling the second flow rate control means based on the temperature of the urea solution in the storage tank.
 20. The reductant supply device according to claim 12, wherein if the cooling water circulation passage, the flow rate control means and the control means are respectively referred to as a first cooling water circulation passage, first flow rate control means and first control means, the reductant supply device further comprising: a second cooling water circulation passage that circulates at least part of the cooling water of the internal combustion engine in order to adjust a temperature of the urea solution in a storage tank that stores the urea solution; second flow rate control means for adjusting a flow rate of the cooling water that flows through the second cooling water circulation passage; and second control means for controlling the second flow rate control means based on the temperature of the urea solution in the storage tank.
 21. The reductant supply device according to claim 13, wherein if the cooling water circulation passage, the flow rate control means and the control means are respectively referred to as a first cooling water circulation passage, first flow rate control means and first control means, the reductant supply device further comprising: a second cooling water circulation passage that circulates at least part of the cooling water of the internal combustion engine in order to adjust a temperature of the urea solution in a storage tank that stores the urea solution; second flow rate control means for adjusting a flow rate of the cooling water that flows through the second cooling water circulation passage; and second control means for controlling the second flow rate control means based on the temperature of the urea solution in the storage tank.
 22. The reductant supply device according to claim 14, wherein if the cooling water circulation passage, the flow rate control means and the control means are respectively referred to as a first cooling water circulation passage, first flow rate control means and first control means, the reductant supply device further comprising: a second cooling water circulation passage that circulates at least part of the cooling water of the internal combustion engine in order to adjust a temperature of the urea solution in a storage tank that stores the urea solution; second flow rate control means for adjusting a flow rate of the cooling water that flows through the second cooling water circulation passage; and second control means for controlling the second flow rate control means based on the temperature of the urea solution in the storage tank.
 23. A control method for a reductant supply device which is used in an exhaust gas purification device that injects and supplies a liquid reductant to an exhaust gas upstream side of a reduction catalyst disposed in an exhaust gas passage of an internal combustion engine, and that reduces and purifies nitrogen oxides contained in exhaust gas using the reduction catalyst, the reductant supply device having a reductant injection valve that is fixed to an exhaust pipe on the exhaust gas upstream side of the reduction catalyst, the control method comprising: cooling the reductant injection valve by circulating at least part of cooling water of the internal combustion engine; detecting a temperature of the reductant injection valve; and controlling a flow rate of the cooling water such that a temperature of a fuel control valve is maintained in a predetermined range. 